1. Field of the Invention:
The present invention concerns a device for the production and modulation of colored light by means of an electrode-coated liquid-crystal cell combined with a polarizing arrangement and a reflector, the liquid crystal having a helical twisted structure in the direction perpendicular to the electrodes.
2. Description of the Prior Art:
Devices of the type mentioned above are already known from German Pat. No. 2,158,563. Essentially they consist of a liquid crystal situated between two transparent electrodes, the molecules of the liquid crystal lying parallel to one another at the electrode surfaces and assuming upon 90.degree. rotation of one electrode with respect to the other a helical twisted molecular structure along the normal to the electrode surfaces, and a polarizing arrangement composed of two crossed, linear polarizers located in front of and behind the liquid crystal, together with a reflector on the back of the rear polarizer. With the polarizers parallel to one another and the polarizing direction of the front one parallel or perpendicular to the preferred direction of the liquid-crystal molecules adhering to the front electrode, incident white light is rotated through 90.degree. by the optical rotatory power of the twisted liquid-crystal layer and is therefore absorbed by the rear polarizer. Upon application of a suitable voltage to the electrodes, the liquid crystal loses its optical rotatory power. The incident polarized light leaves the cell with unchanged polarization direction and, after reflection and re-transmission through the cell and the polarizing arrangement, reaches the observer.
A disadvantage of such devices is that the brightness of the reflected light often is not sufficient since the light must pass through polarizing sheets four times and is greatly weakened by absorption in the process. As is known, polarizers not only weaken the intensity of incident unpolarized light by absorption of the perpendicular component of the field vector, but also absorb a significant portion of the light having a parallel field vector since polarizing films are not optically ideal, so that such devices exhibit only relatively low-intensity display surfaces.